"all the fields in a database"? that seems very extreme as a database can consist of many records in many tables in many files.
Do you, instead, want to lock all the fields for a specific record in one table?
That can be done with record level access control. See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a detailed description of how to set this up.